1. Field of the Invention
The present invention relates to an optical scanning apparatus, and more particularly, to an optical scanning apparatus suitable for an image forming apparatus such as a laser beam printer, a digital copying machine or a multifunction printer, which employs an electrophotography process.
2. Description of the Related Art
A conventional multibeam optical scanning apparatus that forms spots of a plurality of light beams emitted from a light source (multibeam light source), which includes a plurality of light emitting portions (light emitting points), on a surface of a photosensitive drum (photosensitive member) serving as a surface to be scanned has been proposed in Japanese Patent Application Laid-Open No. 2008-225060. In such a multibeam optical scanning apparatus, a specific light beam (reference light beam) among a plurality of light beams is used for detecting a writing start position. For the other light beams that are not used for detecting the writing start position, times between reflection at a plurality of deflection surfaces (reflection surfaces) of a deflecting device and arrival at the surface to be scanned are measured. An arrival timing of each of the light beams is obtained based on a time when the reference light beam is detected, and a writing start timing of each of the light beams is determined based on an average value of the arrival timings with respect to all the deflection surfaces.
However, in the optical scanning apparatus described in Japanese Patent Application Laid-Open No. 2008-225060, the plurality of light emitting portions of the multibeam light source have different positions in the sub-scanning direction, thereby causing the following problem. That is, an image defect is generated due to different surface accuracy between the plurality of deflection surfaces of the deflecting device. Specifically, each of the deflection surfaces is supposed to be formed as an ideal planar surface (rectangle), but in practice, the deflection surface may be formed as a surface having a so-called distortion in which, for example, two vertices on a diagonal line among four vertices of the rectangle are slightly displaced in the same direction.
When the multibeam emitted from the plurality of light emitting portions enters the deflection surface having such a distortion, reflection angles of the beams at the deflection surface differ from each other depending on an incident position of each beam in the sub-scanning direction. This causes each spot irradiation position (scanning start position) of the multibeam on the surface to be scanned to be shifted from its original position in the main scanning direction. In the optical scanning apparatus described in Japanese Patent Application Laid-Open No. 2008-225060, the average value of the arrival timings with respect to each of the deflection surfaces is obtained, and hence there is no consideration on the problem of the distortion generated on each of the plurality of deflection surfaces.
In order to solve the above-mentioned problem, a method of using a deflecting device including a deflection surface having high surface accuracy can be conceived. However, such a deflecting device increases cost.